1. Field of the Invention
The present invention relates to fiber-optic communications, and, in particular, to detecting the existence of data channels carried by fiber-optic systems.
2. Description of the Related Art
Data may be transmitted through a particular communications medium, such as a fiber-optic cable, by way of multiple data channels, where each channel carries a data signal at a particular carrier frequency. Because each frequency is associated with a different wavelength, such systems are multi-wavelength systems, which are sometimes referred to as wavelength division multiplexed (WDM) systems or networks.
WDM systems may use a fiber-optic cable as the communications medium. The fiber-optic cable carries light signals from a transmitter to one or more receivers. These light signals are sent on one or more of the multiple channels, each channel being defined by a different light wavelength. The signal transmitted by light of a particular wavelength on a particular channel may be a digital signal, in which case light is transmitted to indicate a binary "1," and not transmitted to indicate a binary "0." Thus, channel 1 may carry light of a first frequency (or wavelength), channel 2 light of a second frequency, and so forth, each frequency of light turning on and off independently to represent 1s and 0s. Such carrier frequencies are typically in the hundreds of TeraHertz, such as 193.48622 THz. The presence of light at the first frequency indicates a "1" on channel 1, while its absence indicates a "0". At such frequencies, such channels may carry binary data on the order of 2.5 Gigabits/sec.
In such systems, signals may be transmitted over relatively long distances, for example from New York City to Washington, D.C. Optical amplifiers (OAs) are used along the route to buffer the signals, which otherwise would degrade. These OAs may be placed, for example, every 20 km or so. One type of OA is an Erbium-doped fiber amplifier (EDFA). An EDFA comprises a portion of a fiber-optic cable doped with Erbium ions and a laser diode. The light from the laser diode excites the Erbium. The light signals of the channels carried by the fiber-optic cable enter the EDFA portion of the cable and stimulate photon emission by the excited Erbium ions, which effectively boosts (i.e. amplifies) the light signals. Current EDFAs can amplify input light that has a wavelength between 1550-1560 nanometers. This 10 nanometer bandwidth provides over a TeraHertz of frequency range, which is sufficient to support an eight-channel WDM system.
At a point somewhere along the communications medium, such as at a tap node, receiver, or other station of the system, it is often desirable to determine whether signals are being transmitted on predefined channels of the system, and what the power level of the signal being transmitted on each channel is. If, for example, a channel 1 signal is supposed to be present, a failure to detect a signal on channel 1 at a given point in the system can be used for diagnostic purposes. For example, a failure to detect a signal on one or more channels may indicate that one of the EDFAs is malfunctioning, if that channel is supposed to be present. Similarly, detection of a lower than normal power level can indicate some problem in the system that is affecting the transmission of data on channel 1. When a signal is being transmitted on a particular channel and is present at a further point in the system, the channel may be said to exist at that point in the system. The channel may also be said to have a given power level at that point in the system, corresponding to the average power level of the signal carried by that channel.
One way to detect the existence of a channel is to utilize a standard photodetector and receiver approach that would normally be used for receiving and decoding the information carried by the channel itself. However, this approach is often undesirable because it can require relatively complex or expensive components that may not otherwise be required at that node of the system. While certain nodes may require such equipment in order to actually decode the transmitted data, it can be inefficient to require the use of such sophisticated, potentially expensive, and complex devices whenever it is desired to merely detect the existence or measure the power of a channel.
Another way to detect the existence of a channel at a particular point in the system is to utilize amplitude-modulated (AM) tones of a lower frequency than the carrier frequency of the channel. In such an approach, a different low-frequency AM tone is placed on each channel at the transmitter side. These tones may then be measured at a particular station or node of the system, with devices suitable for detecting signals at these much lower frequencies. A typical AM tone for a given channel may be a 1 KHz tone, which causes the average power level of the channel to vary at 1 KHz. Because the AM tone is added to the light signal of a given channel, the AM tone's presence and power is indicative of the channel's existence and power level. One drawback of such an approach, however, is that the AM components of the tones can mix together significantly in the EDFAs of the fiber-optic cable. Further, such AM tones can also interfere with the data in the channels. One reason for such problems is that lower-frequency signals, such as the AM tones, can deplete the gain of typical EDFAs, causing cross-talk between channels because of the time lag in changes of the EDFA's gain.
There is a need, therefore, for methods and apparatuses for detecting the presence, and measuring the power, of modulation of channels.
Further objects and advantages of this invention will become apparent from the detailed description which follows.